Florida Geological Survey


2 and 3). Glauconite, clay, and collophane are common accessory minerals within an interbedded sequence of wackestone and dolostone of the uppermost Oldsmar. The gamma-ray log, in response to this mineralogy, shows a distinct increase of gamma-ray activity across the zone which is correlative throughout the study area. The sonic log response across this interval, although not directly related to this mineralogy, is correlative to a lesser degree among all the injection wells. Sonic log interval transit times are highly variable through the uppermost Oldsmar, apparently reflecting porosity differences between the more porous limestones (approximately 30 percent porosity) and less porous dolostones (approximately 15 percent porosity) of this interbedded interval.

 Sonic log response corresponds well to actual lithology in sequences consisting of lower porosity dolostones interbedded with higher porosity limestones. Dolostones in such cases have sonic log curves that peak in the low porosity direction (generally 20 percent or less porosity). Sonic log curves in limestones, on the other hand, peak in the high porosity direction (30 percent or greater). This relationship is especially true of upper Avon Park Formation sections in Brevard County. The Merritt Island sonic log and lithostratigraphic section from 400 to 800 feet (BLS) in the upper Avon Park Formation offers the best example of this property (Figure 11).

 Induction resistivity logs can be useful in distinguishing relatively low porosity zones from relatively high porosity zones within the lower Floridan aquifer system. Low porosity, saltwater saturated carbonates are highly resistive and are typically indicative of dense dolostones or possibly calcite spar cemented limestones. Highly porous, saltwater-saturated carbonates have low resistivities and are generally indicative of porous dolostone or moderately to poorly-indurated limestone. Induction log resistivities across cavernous zones are extremely low and approach, if not equal, that of the formation water alone.


 Several sections of the Floridan aquifer system have gamma-ray signatures that are correlative throughout Brevard County and serve as excellent datums for stratigraphic and structural analyses. In addition to the uppermost Oldsmar glauconitic zone, the lower Oldsmar Formation contains several correlative gamma-ray marker horizons including the "C" marker bed (Plates 1 and 2). The "B" marker bed (Plates 1 and 2) roughly divides the Avon Park Formation into upper and lower sections. The uppermost Avon Park, from the top down to the "A" marker bed (Plates 1 and 2), has highly correlative gammaray character.

 DOLOMITIZATION IN THE LOWER
 FLORIDAN AQUIFER SYSTEM

 Several investigations into the nature of dolomitization within the Floridan aquifer system have been conducted. Studies done by Hanshaw et al., (1971), Cander (1991), Randazzo and Hickey, (1978), Randazzo and Cook, (1987) and Randazzo et al., (1977), focused on the lower Floridan aquifer system and are summarized in the following discussion.

 Hanshaw et al., (1971) hypothesized a mixing zone dolomitization model for Floridan aquifer system dolostones of regional extent. In their model, dolomitization occurs in brackish waters formed where freshwater mixes with seawater along coastal areas or subsurface brines further inland (Hanshaw et al., 1971). Circulating ground water having a Mg/Ca ratio > 1 is the driving force for dolomitization in the mixing zone (Hanshaw et al., 1971). Thermal convection of saltwater within the Florida Platform, as proposed by Kohout (1965), could provide the circulation and mixing mechanism for dolomitization of much of the Floridan aquifer system carbonates (Hanshaw et al., 1971). The lateral and vertical movement of the saltwater-freshwater interface due to sea-level variations, climatic changes, and/or platform uplift or subsidence has also facilitated dolomitization within much of